Cathode-ray system with bright and dark marker indications



C-R. BEAM CURRENT Aug. 12, 1947. D, E. KENYON 2,425,330

CATHODE RAY SYSTEM WITH BRIQHT AND DARK MARKER INDICATIONS I FiledJune 22, 1945 FlG.l V52] VIDEO CATHODE FOELOWER NETWOR RADIO OBJECT DETECTOR IMITER mcnnomzuc CONNECTION BRIGHT MARKER GENERATOR i I- Z III I: I D U I x 94 5 i '4 CD I v MICROSECONDS 3 2 3 5, HI I ///2. 3 x 9'7 107 99, .mg 0 n i '9) I. 94 II II I l l l I A l l I J -l60 9o .0 +90 n AZIMUTH H S ATTORNEY MFCROSECONDS YON Patented Aug. 12, 1947 UNITED STATES EPATEN orrice CATHODE-RAY SYSTEM WITH BRIGHT AND DARK MARKER INDICATIONS David E. Kenyon, Smithtown, N. Y., assignor to Sperry Gyroscope Company, Inc., a corporation of New York Application June 22, 1943, Serial No. 491,766

14 Claims.

improvement of pattern-tracing apparatus of the i type employing a defiectible beam of radiant energy impinging on a screen, and an object is to increase the clarity of the indications.

A further object of my invention is to prevent confusion between background indications and marker indications. More specifically, an object of my invention is to make scale marker indications visible without regard to the existence of extraneous or background indications or'the extent of video indications.

It is an object of my invention to provide a method for drawing a dark line through a luminous area on the screen of a cathode ray tube. It is also generally an object of my invention to provide a method for producing scale division lines on a cathode ray screen or the like and more particularly to provide a method to make the lines visible whether the screen is brightly illuminated or dark.

In certain types of object locating apparatus cathode ray tubes with fluorescent screens are employed as indicators. Graphic indications are produced on the fluorescent screen by the deflection of the cathode ray beam of the tube. It is considered preferable to utilize characteristic bright spots on the fluorescent screen instead of an external or printed scale for determining distances along an axis. Such marks are produced by means of bright marker generators acting upon the cathode ray beam and producing bright spots on the screen at predetermined intervals corresponding to deflection of the beam along the axis in question.

Under some conditions, however, difiiculty is encountered from certain types of signal indications, such as ground reflections or very extensive target indications, for example, which produce a large substantially continuous area of illuminated spots, and thus make it difiicult to distinguish the bright marker scale indications from the luminous area.

A specific object is to produce what I 1 to be produced only when strong signals are being It is accordingly an object of my invention to provide means for substituting black marker scale indications for bright marker indications under conditions when such interfering luminous areas are produced.

A further object is to provide an arrangement whereby marker indications are automatically changed from bright marker to black marker indications, and vice versa, as the operating conditions change, so as to make one type of marker indication or the other more readily visible.

Other and further objects and advantages will become apparent as the description proceeds.

In carrying out my invention in accordance with a preferred form thereof, a cathode ray tube is arranged so as to be responsive to a video receiver for producing signal indications on its fluorescent screen, and also responsive to a black marker generator for producing horizontal scale marker indications. on the fluorescent screen. In

addition, means are provided for deenergizing the cathode ray tube or rendering the tube ineffective to produce luminous spots, and an inverter is interposed between the bright marker generator and such tube deenergizing means for deenergizing the tube and thereby producing black marks at the cathoderay beam positions at which bright marks would ordinarily be produced by the bright marker generator.

, In order to cause the black marker indications received, a control device is interposed between the video receiver and the inverter for rendering the inverter, effective only during such times as signals above a predetermined level are received. A better understanding of the invention will be aiforded by the following detailed description considered in connection with the accompanying drawings, wherein Fig. 1 is a circuit diagram, partially schematic, of one embodiment of my invention.'

Fig. 2 is a diagram illustrating the appearance of the screen of the cathode ray indicator tube under certain illustrative conditions in which several islands are indicated as well as objects to be located.

Fig. 3 is a graph of C. R. beam current plotted against time for the period of vertical scan producing a single verticalline (3--3) of. the pattern of Fig. 2 when no black markers are employed.

Fig. 4 is a graph corresponding to Fig. 3 with theeffect of black markers shown.

I Like reference characters are utilized throughout the drawings to designate like parts.

In microwave pulsetype object locators, ob-

jects are detected by directing a series of pulses of microwave radiant energy in the direction in which the possible existence of an object is suspected and observing whether or not any pulses are returned, thus indicating the presence of an object serving to reflect the transmitted pulses. The apparatus for transmitting the microwave pulses and for receiving the reflected pulses is pivotally or rotatably mounted so as to scan an area which is to be patrolled, or in which objects are to be located. The location of the object is then determined .bydetermin'ing the angular position of the scanner atthe instant when the reflections are received.

For indicating the location, an ,indicator of the cathode ray tube type may be employed in which reflected pulses produce luminous marks on the screen and the cathode ray beam is swept across the screen both vertically and horizontally in order to scan the entire surface. The beam is swept horizontally across the screen in accordance with the sweeping or scanning motion of the scanner so that .the positions on the screen of'the indications of reflected signalsserve as indications of the angular positions of the detected objects. The beam is also sweptvertically alqn 'the screen to represent distance or range.

In order that stray magnetic fields, and the ,position of the apparatus with respecttothe earths magnetic field-will noteffect thescalecalibration by virtue of magnetic, action on the cathode-ray .beam, aswell as for other reasons, electrically produced range marker indications are used on the scaleflinstead ,of ,an 1 external or permanently printed or engraved scale on the screen.

Such ,a system is represented schematically in Big. 1 .of the drawings, vwhereinall of the apparatus for producing,andtransmitting microwavepulses,andconverting reflected pulses into video signals is representedschematically byv a box Il,.one, portion- IQ of which represents a receiver from which video signals-are received :through ,an output ,channel 13 represented ,schematicallyby,a-single line. The radio object detector apparatus l I is represented-as. including ,aparabolic. antenna or radiator and scanner l4, shown, schematically as having. a pivot axis 15 so that the scannerl4 maybemoved-back and forth in the directions indicatedbythe artosweep along the desired length of the horizon. For producing indications of the output of the receiver l-Z, oneor more cathode ray tubes such ,asa tube 18 may be provided, according to the number of indicating stations desired.

For controlling the deflection of the cathode ray beam in-one ,axis, for examplethevertical axis, to represent range, each of the tubes I8 is -provided with-a: cathode:ray beam deflecting circuit including electrostatic deflection plates 'orimagnetic deflection coils 20, and a synchronized sweep Wave generator (not shown) is arranged to actuate such deflection circuits. Each tubehas aseco-nd set of electrostatic deflection plates-or cells '21 coupled in a manner ,(not

shown) to the mechanical motion of the scanner 14 for'sweeping the cathode ray beam alonga second axis, inthis case the horizontal axis, so

.that thedistance along the horizontal scale of the cathode ray screen indicates angular position of the scanner and therefore angular position of an object detected. by a reflected pulse.

For producing a scale on the cathode ray screen, a standard bright marker generator 24 is provided. The standard bright marker generator 24 is synchronized with the transmitter in the radio object detector ll through a suitable electrical coupling, schematically represented by a line 25, so as to produce voltage impulses at regular intervals after each train of microwave oscillations is emitted by the scanner I4.

For actuating the cathode ray tube IS in accordance with output of the receiver I2 from the channel [3, a connection is provided including a video cathode follower network 26, limiter 21, a cable 28, and a brilliance control potentiometer 3|.

' *For coupling the bright marker generator 24 to thev cathode ray tube 18 an electrical connection 33, a cathode followerstage 34, and a coupling condenser 35 are provided together with the previously mentionedelements 21, 28 and 3|.

For producing black marker indications instead -of bright marker indications an inverter 36 is provided which is interposed between the bright marker generator 24 and the tube 18. For con- ,trolling the inverter 36. in accordance with out- -putfrom the receiver [2 acontrol device -3l,is provided.

The cathode ray, tube [8 comprisesan evacuated envelope containing a fluorescent screen 38, one or more anodes and focussing electrodes or the like ,(not shown), a cathode 39,.a control grid 4l, and a second control element 42, which, may take the form of an intensifier or screen grid or other suitable means for deenergizing or rendering the tube ineffective.v The .brightnesscontrol potentiometer 3| comprisesa resistor43 connected from, the control gridfil to a point. of ne ative potential or ground and .a slideable tap. or brush 44. The brush 44 is connected to the endof the cables 28, which preferably has a grounded sheath.

For controlling the background level. of the cathode ray trace a potentiometer 45 is provided which includes a t ap connected to cathode ,39 and a resistor connected betweenground and the end of a grounded sheath cable 46,.which is connected through a conductor 47 to .a gate controlter- 'minal 48 inthe radio object detector I I. v

The inverter 36 is shown in the formofa triode and may be one element of a .twin triode, the other element of which comprises the control device 31.

As shown, the inverter 33 includes an anode 49, a cathode 5!, and a control electrode or grid 52. The control electrode, 52 is coupled by means of a condenser 53 to the output channel 33. of the bright marker generator 24. .For negativelybiasi-ng the grid 52 and adjustably. controlling black marker gain, a potentiometer 54 is provided comprising a resistor 55 connected between ground and a source of negative potential. 56, and having a sliding brush .5! connected to the grid 52 through a grid leak resistor 58.

Forcoupling the inverter anode 49 to the intensifier grid 42 of the cathode ray tube, a ,cou-

pling condenser 59 is. provided and the intensifier grid 42 is connected to thecondenser 59 through cablestl andconductors 62 and63. For normal- 1Y.maintaining theintensifier grid:42 atra predetermined level of positive potential, a positive voltage source 64 is provided; and for dropping the voltage of the source34 and coupling the tube 36 to grid 42 a resistor 65 is connected between, the

positive terminal 64 and the intensifier grid connection 62.

The inverter tube 3t may be provided with a conventional load resistor 66 connected to the positive terminal of source 64.

The control device 31 comprises an anode 68, a cathode E9, and a control electrode or grid II. The grid H is coupled through a coupling condenser [2 to the output channel l3 of the receiver l2. The cathode 63 is grounded and thus connected to the negative terminal of the anode voltage supply'source 64 and the anode 38 is connected to the cathode 5| of the inverter triode 35, so that the supply source 64 supplies the tubes 36 and 31 in series.

For negatively biasing the control grid II, a source of negative potential 13 is provided to which the grid II is connected through a grid leak resistor I4. In order to prevent the control grid 1| from ever being driven more negative than the potential of the negative bias source '13, by negative peaks of the video output from the receiver I2, a direct current restorer is provided, which may take the form of a rectifier I5 connected across the grid leak resistor M. The rectifier 75 may consist of a diode, or, as shown, a triode with the anode and grid connected together to form a diode.

It will be observed that the rectifier 15 has an anode I6 connected to the negative end of resistor 14 and a cathode I! connected to the positive end thereof.

The cathode follower stage 34 for coupling the marker generator 24 to the cathode ray tube grids AI may comprise a triode conventionally connected with an output resistor I8 in the cathode lead thereof, the coupling condenser 35 being connected between the cathode end of the resistor 18 and the connection 29 from the video cathode follower network 25.

For controlling the bright marker gain the cathode follower 3 is adjustably biased. An adjustable bias source 79 is provided consisting of a resistor 8! connected between ground and a source of negative potential 53, and an adjustable tap or brush 82 connected through a resistor 83 to a control electrode or grid 84 of the cathode follower tube 34. The control grid 84 is coupled through a condenser 85 to the output channel 33 of the bright marker generator 24.

A pattern is produced on the screen 38 of the cathode ray tube by sweeping a cathode ray beam across the screen with both horizontal and vertical scanning motions and energizing the tube sufficiently to illuminate the screen only when the beam is in the position at which the illumination is desired on the screen. For example, as illustrated in Fig. 2, the screen 33 is represented as having a rectangular area 9| within which a pattern or patterns may be produced representing targets or objects intercepted by a microwave beam transmitted from the microwave pulse scanner I4. Certain areas 92, shown as cross-hatched represent targets, islands, ships, airplanes, icebergs, or other objects within the range of detection of the apparatus of Fig. 1. The areas 92 are luminous, whereas the remainder of the screen within the rectangle 9| is dark or has only a relatively faint background illumination according to the setting of the potentiometer 45 (Fig.1). The beam is swept horizontally at a relatively slow rate by the horizontal deflection coil 2| and is swept vertically at a relatively high rate by the vertical deflection coil 20. 1

The luminous areas result from the fact that during the instants of timethat the beam deflection conditions are correct for electron beam impingement on the portion of the screen 38 of the cathode ray tube I8 Within these areas, the control grid 4| is energized sufficiently by the video output of the receiver l2 to illuminate the screen 38. Thus, the pattern is produced on the screen by the beam making a series of vertical traces across the screen and either illuminating the screen or leaving it dark according to the voltage applied to the control grids M. For example, when the horizontal sweep has carried the beam to the azimuth position represented by the arrows'3'3, the voltage applied to the grids 4| varies with time in the manner illustrated by the graph of Fig. 3.

The horizontal or time axis of Fig. 3 corresponds to the vertical or distance axis of Fig. 2, since the time required for energy to travel to a reflecting object and return is proportional to the distance to the object from the scanner l4. Unless the pulse produced by the transmitter in the radio object detector II is cut out from the receiver, the receiver I2 will produce a video output signal when the pulse is transmitted. Such a direct pick-up ishowever assumed to have been eliminated from the video input to the cathode ray tube by the provision of a cathode ray gate from the terminal 48 and therefore no corresponding indication appears in Figs. 2-4.

If the apparatus is used at sea, a few microseconds after the emission of a pulse or train of microwave oscillations, the video voltage will rise momentarily as shown at 94 in Fig. 3, representing a train of microwave pulses reflected from the surface of the ocean immediately in front of the apparatus I I of Fig. 1. This is indicated in Fig. 2 by a dotted line 95 and called the sea return. Since the receiver I2 serves as a demodulator, only the modulation envelope of the train of microwave oscillations appears in the wave form 94.

If an island of considerable extent covering a distance from about 1 miles to 2% miles from the transmitter, for example, intercepts the train of emitted microwave oscillations, corresponding signals will be reflected from the island for a period of time. Thus when the transmitted oscillations first strike the closest portion ofv the island, reflected signals will start returning toward the radio object detector I I and be picked up by the scanner I4. As the transmitted oscillations reach'more and more distant portions of such an island, additional reflections start returning toward the radio object detector, but owing to the time delay involved in traveling to and from the successively more distant parts of the island, the reflected oscillations will continue to be received by the radio object detector I I for an appreciable period of time. When the microwave radio frequency voltage is demodulated by the receiver I2, the video voltage output of the receiver I2 will have the form shown by the portion 96 of the curve in Fig. 3. The voltage 96 appears as a line 91, 98 in one of the areas 92A of Fig. 2.

As represented in Fig. 3, this video voltage persists from an instant about 12 microseconds after the transmitted pulse to about 35 microseconds after the transmitted pulse. This time interval represents the difierence between twice the length of time for amicrowave signal to traverse the distance from the transmitter to the nearest point of the island and twice the time required to traverse the distance: to the farthest point of th 7 island delineated by the line 91, 98 in Fig. 2. The length of the line 91, 98 therefore represents the length of the island, and the distances from the zero line I9 to the points 91 and 98 in Fig. 2 represent the distances from the radio object detector II to the nearest and farthest points, respectively, of the island which has been detected. In a similar manner, if a second island is intercepted by the transmitted microwave beam while the cathode ray beam'is still being swept vertically along the line 33 of Fig.2, the video V01t-. age will'rise again, as represented by the portion 99 of the curve of Fig. 3, and a bright line IIJI, I02 will be traced on the screen 38 of Fig. 2.

When the cathode ram beam makes its next vertical sweep, the horizontal sweep coils will have moved the beam slightly to. one side., Consequently, successive vertical sweeps produce lines corresponding to the lines .91, 98 and- IOI, I92 50 as to produce the luminous areas 92A and 923 in Fig. 2. Other islands or detected objects are indicated by the other illuminated areas 92 of Fig. 2; It will'be understood that if a navigator'is supplied with a chart of the portion of the sea in which heis navigating, he W111 recognize the bright areas corresponding to islands, and if any additional spot not corresponding to charted obstructions is observed, such as a spot 92C, for example, the navigator will recognize it as an indication of the presence of an obstruction to navigation; a hostile ship, aircraft, or the like.

The'screen 38 is provided with a horizontal scale graduated in degrees to designate angles in azimuth at which the various detected obstructions are located and is also provided with a vertical scale graduated in terms of range. For example, therange gradu'ations may cover the region from zero to five miles where the length of the horizontal axis of Fig. 3 covers a time.

duration of approximately 60 microseconds, the time required for radio energy to travel to and from an object five miles distant.

In order to facilitate reading the range, it is desirable to produce on the screen 38 horizontal graduation lines or range markers I04. These may be produced by a bright marker generator such as shown at 24 inFig. 1, which produces momentary voltage impulses such as those indicated by the reference numerals I95, I06, I01, I98 and I09 in Fig. 3. Ordinarily, the receiver I2 includes suitable limiting circuits so that the voltage curve of Fig. 3 has a substantially constant maximum value producing a fiat top or series of flat tops. The bright marker generator 24 may be so connected that the voltage impulses I95, and so forth, are superimposed on the video output of the channel I3 of the receiver I2. However, even if this is done, the added brightness of the screen may not be sufficient to'produce clear range marker lines through extensive illuminated areas, such as the areas 92A and 92D.

Thus, in Fig. 3 the marker impulses I05, I01 and I99 fall in clear-areas so as to produce bright spots which result in luminous lines I04 being drawn horizontally across the screen 38 as the beam is swept horizontally. It will be understood that the persistence of vision or of the fluorescent screen material causes the marker lines to appear as continuous lines, though the bright-spot is actually at only one point of the screen at a given instant. On the other hand, the marker impulses I06 and I98 which occur during: the long. voltage pulse areas 96 and 99 of the curve of'Fig'. 3Jare substantially obliterated 8 and are therefore represented by dotted lines in Fig. 3. The limiter 21 gives the waves of Fig. 3 a uniform maximum height.

In order to produce range markers which will be distinguishable from the luminous areas 92 when they fall within the areas 92, the apparatus may be'modified so as to produce a curve such as shown in Fig. 4, for example. In this case the cathode ray tube is so operated that, in effect, the bright markers I06 and I98 of Fig. 3 are reversed, as represented by negative impulses III and H2 in Fig. 4, which decrease the C. R. beam current and therefore produce black markers instead of bright markers whenever the bright marker impulses occur during video voltage impulses, such as the impulses 96 and 99. It will be understood that Fig. 4 is symbolical, and that my invention is not limited to producing black markers by actually decreasing the video voltage to reduce C. R. beam current. As illustrated in Fig. 1, such an effect may be produced instead by deenergizing the cathode ray tube I8 at given instants when black markers are to be produced by impressing negative black marker impulses on the screen grid or intensifier grid 42 instead of superimposing such impulses on the control grid 4!. Nevertheless, the effect represented by Fig. 4 is accomplished because at the instants of the black marker pulses II I and H2, the cathode ray tube is deenergized so as to darken the screen 38 of the tube and thereby cause dark lines H3 and H4 drawn through the luminous areas 92A and 92B, as well as causing similar dark line segments to be drawn through. the other luminous areas when intersected by one of the range markers I04.

Referring to Fig. 1, it will be observed that the bright marker pulses from the bright marker generator 24 are inverted by the tube 36, so that when the tube 36 is functioning a negative impulse is applied to the intensifier grid 42 at each instant corresponding to a bright marker indication. Since the functioning of the cathode ray tube depends upon energization by intensifier grid 42, the deenergization of this grid deenergizes thetube or renders it ineffective so that the cathode ray beam is momentarily cut off and black marker indications instead of bright marker indications are produced along the range markers I94 in Fig. 2.

Although the invention has been described in connection with a system having means for producing range markers and not producing azimuth markers it will be understood that my invention is not limited thereto and embraces the use of black markers instead of bright markers when required by operating conditions regardless of the dimensions indicated or the use to which the bright markers are put.

The control device 31 serves to switch the apparatus automatically from bright marker to black marker indications and vice versa as operating conditions change to require a change in markers. Thus, whenever video signals are received, the control tube becomes conducting and provides a ground connection for the cathode SI of the inverter tube rendering the inverter tube effective and causing the black marker pulses to be supplied to the intensifier grids 42.

On the other hand, when there are no video signals or when the video signals are below the level represented by the bias of the control tube 311; indicating that relatively little energy is being received from the receiver I2, the tube 31 has substantially infinite impedance, Under these aroused donditions, the cathode 5| of the inverter tube 36 floats at relatively high potential and no pulses aretransmitted t the intensifier grid 42.

The video-cathode follower network 26 provides low impedance input to the cathode'ray grid circuits and prevents reaction upon the receiver I2 and interference with the control tube 31 by pulses from the bright marker generator 24. Likewise the cathode follower stage 34 provides low output impedance.-. 1..

1 In accordance with the provisions of the. patent statutes I have described the principle of operation of my invention together with the apparatus, which I now consider to represent the best embodiment thereof, but I desire to have it understood that the apparatus shown is only illustrative and that the invention may be carried out by other means.

What is claimed is:

. 1. An object locator comprising, in combination with a radio detection system, a radio receiver, a cathode ray oscilloscope responsive to the receiver, a standard marker generator for supplying the cathode ray oscilloscope with scale marker indications, an inverter coupled to said marker generator, and means coupling the oathode ray oscilloscope to said inverter for rendering the cathode ray oscilloscope momentarily inefiective in response to an output signal from said generator during a period of receiver output.

2. An object locator comprising, incombination with a radio detection system, a radio receiver, a cathode ray oscilloscope responsive to the receiver, a standard marker generator for supplying the cathode ray oscilloscope with scale marker indications, an inverter coupled to said marker generator, means operable by said inverter for rendering said oscilloscope ineffective, and means responsive to the receiver output for controlling said inverter and rendering it efiective only during the existence of output from the receiver.

3. A black marker generator for use with a radio detector system subject to marker masking indications having a receiver, a cathode ray oscilloscope responsive to the receiver and a standard marker generator supplying modulated impulses to the cathode ray oscilloscope, said black marker generator comprising means responsive to said generator for deenergizing said oscilloscope, and control means interposed between said receiver and said deenergizing means for rendering said latter means efiective only during the existence of output from said receiver.

4. In combination, a cathode ray oscilloscope having a control grid and an intensifier grid, a video receiver coupled to the control grid, a marker generator also coupled to the control grid, control means responsive to receiver output, and an inverter interposed between said control means and said intensifier grid for deenergizing said intensifier grid during the existence of receiver output to produce black marker impulses to contrast with video indications.

5. Apparatus as set forth in claim 4, in which the control means comprises a triode vacuum tube with a control grid, means for biasing said grid beyond negative cut-01f, and means for preventing the application of negative signals to said grid.

6. An object locator comprising a detection system including a radio receiver, a cathode ray tube with a screen adapted to be illuminated at a spot where a cathode ray impinges, a bright marker 10 J generator coupled to said tube, means responsive to said generator for diminishing brightness an-d means responsive to said receiver for controlling saidbrightness-diminishing means.

7. An object locator comprising a detection system, an indicator or tracing means with a screenadapted to be'illuminated along a trace, a bright marker generator coupled to said indicator, means to produce a bright marker trace thereon, for reversing the action of said generator on said indicator to produce a dark marker trace thereon. and means responsive to output from said detection system for controlling said previously mentioned means.

8. An object locator comprising a detection system, an indicator or tracing means with a screen adapted to be illuminated along a trace, said indicator or tracing means being coupled to said detection system for producing areas of contrasting illumination on said screen, a bright marker generator coupled to said indicator to produce a trace thereon, and means for reversing the action of said generator on said indicator, whereby said trace is illuminated in contrast with said screen areas.

9. A black marker generator for a cathode ray tube comprising a bright marker generator coupled to said tube for producing a bright trace therein and an inverter responsive to said bright marker generator and operative on said cathode ray tube for producing a dark trace therein.

10. A black marker generator for a cathode ray tube comprising a pulse generator coupled to said cathode ray tube through an intensifier channel and also through an intensity-reducer channel, and a receiver-responsive control for said generator for rendering said intensity-reducer channel operative in accordance with the received signals.

11. In combination, a cathode ray tube a signal source coupled to said tube for producing a pattern thereon, and a marker generator coupled to said tube for supplying marker signals therein, and means for deenergizing the tube in accordance with simultaneous signals from said marker generator and said signal source.

12. A method of drawing a dark line through an illuminated portion of the screen of a cathode ray tube, which comprises the steps of generating marker impulses synchronized with beam deflection through said line, and deenergizing the cathode ray tube electron beam in response to such impulses.

13. A method of producing scale markers on the screen of a cathode ray tube which will be visible in either dark or illuminated portions of the screen, which method comprises the steps of energizing the cathode ray tube in response to video signals for producing a pattern on the screen, generating marker impulses, applying said impulses to the cathode ray tube for producing bright markers, inverting said bright marker pulses, deenergizing the tube by said inverted pulses to produce black markers, and rendering said inverter effective or ineffective in response to the magnitude of the video input to the tube for preventing the production of bright markers and causing the production of black markers only when the video input exceeds a predetermined level.

14. The method of producing a, luminous pattern on the screen of a cathode ray tube together with scale markers, which method comprises energizing a tube in accordance with video signals 11 12 to be indicated on the screen, generating marker UNITED STATES PATENTS 11npulses, and selectlvely energlzlng or deener- Number Name Date glzlng sald tube 1n response to sand marker 1m- 2 121 359 Luck et a1 June 21 1938 pulses according to whether the video signallin- 2137262 fi' 'g gfi'j Nov; 1.938 put level Is above or below a predetermmed level. 5 I 2,227,197 Percival Dec 31 .1940 V 2,275,460 Page Mar. 10, 1942 DAVID KENYON' 2,285,043 Messner June 2.1942 2 313 967 Read Jr. Mar. 16'1943 RE FERENCES CITED 10 2,227,492 C. L. Faude1l etal. Jan. '7, 1941 The following references are of record in the 2,255,484 R. B. Dome Sept. 9, 1941 fi f t pat nt: 2,255,485 R. B. Dome Sept. 9, 1941 

